Continuous production of sodium dichloroisocyanurate dihydrate

ABSTRACT

THIS INVENTION PROVIDES A CONTINUOUS PROCESS FOR PRODUCING SODIUM DICHLOROISOCYANURATE DIHYDRATE WITHOUT SIGNIFICANT COPRODUCTION OF SODIUM CHLORIDE IN THE SAME AQUEOUS MEDIUM. A MULTISTAGE PROCESS IS PROVIDED EMPLOYING CYANURIC ACID, SODIUM HYDROXIDE, AND CHLORINE OR THEIR EQUIVALENTS AS REACTANTS AND USES A CRITICAL DRYING STEP TO PRODUCE SODIUM DICHLOROISOCYANURATE DIHYDRATE.

A ril 9, 1974 s. BERKOWITZ 3,303,144

CONTINUOUS PRODUCTION OF SODIUM DICHLOROISOCYANURATE DIHYDRATE FiledJune 29, 1972 fieza 3:5 :3 553m MN-155i Q53 3. 2 5 855mm M :5

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United States Patent Office U.S. Cl. 260-248 C 6 Claims ABSTRACT OFTill", DISCLOSURE This invention provides a contlnuotts process forproducing sodium dichioroisocyanurate dihydrate without significantcoproduction of sodium chloride in the same aqueous medittm. Amultistage process is provided employing cyanuric acid. sodiumhydroxide. and chlorine or their equivalents as reactants and uses acritical drying step to produce sodium dichloroisocyanurate dihydrate.

This application is a continuation-in-part of Ser. No. 252.993, filedMay 15. 1972. now abandoned.

This invention concerns sodittm dichloroisoyeanurate dihydrate which isa bleaching compound that has been produced by a process which resultsin sodiam chloride being simultaneously produced as n eoproduct.

The process disclosed in U.S. Pat. 3.035.056 produces sodiumdiehloroisocyanurate dihydrate by the chlorination oftrisotliumisocyanurate with chlorine. The production of sodiumdichioroisocyanurate dihydrate from trisodiumisocyaaurate and chlorineproceeds according to the reaction:

This reaction results in the production of both sodiumdichloroisoeyanurate and sodium chloride. The presence of sodiumchloride in a sodium diehlorolsocyanurate is undesirable in that itinterferes with stability of the dichloroisocyanurate dihydrate. inorder to continuously obtain a product that is substantially tree ofsodium chloride substantial quantities of water must be used in thereaction mixture. However. if the aqueous medium is recycled afterseparation of the sodium dichlorolsocyanurate dihydrate, then sodiumchloride will build-up in the aqueous medittm regardless of the amountof water initially present because sodium chloride is continuouslyproduced at a ratio of two moles of sodium chloride per mole sodiumdichlorolsocyanurale dihydrate. Therefore large quantities of water arerequired and the aqueous medium cannot be readily recycled.

Another method for producing sodium dichloroisocyanurnte dihydratedisclosed in British Pat. 923,147, which corresponds to U.S. Pat.3,035.056, comprises chlorinuting one mole of trlsodium cyanurnte withtwo moles of trichloroisocyanuric acid. While such a reaction does notcoproduce significant amounts of sodium chloride. it requires a separatesource of trichioroisocyanuric acid in order to obtain the requiredreactant for the process.

Anhydrous sodium dlchloroisocyanurate is produced according to theprocess described in U.S. Pat. 3,270,017 or similar processes.

Patented Apr. 9, 1974 Cynauric acid is commonly represented as existingin two tautomeric forms as follows:

The terms dichloroisocyanurie acid and dichioroisocyanurate refer to theacid and salt respectively in either tautomeric form.

This invention provides a continuous process for roducing free-flowing.particulate sodium dichloroisocyanurate dihydrate without thecoproduction of sodium chlo ride in the same aqueous medium comprisingreacting cyanuric acid. sodium hydroxide (and/or sodium hypochlorite),and chlorine in a first aqueous medium to produce a dichloroisocyanuricacid slurry, separating the solid dichloroisocyunuric acid frotu theslurry, neutralizlag the solid dichloroisocyanuric acid with sodiumhydroxide in a second aqueous medium to form sodium dlchioroisocyanuratedihydrate particles, separating the sodium dichloroisocyanuratedihydrate particles from the second aqueous medium. drying the wetsodium tiichloroisocyanurate dihydrate particles at a temperature fromC. to 50' C. to produce essentially dry sodium dichloroisocyanuratcdihydrate and recycling the second aqueous medium preferably to theneutralizing stop.

The figure is a schematic representation of the invention. it depictsthe process for continuously producing sodium dichloroisocyanuratedihydrate from chlorine, cyanuric acid and sodium hydroxide.

The process provided by this invention can be dclcribcd in greaterdetail with reference to the figure. Cyanuric acid, sodium hydroxide,and chlorine are reacted in ehlorinator 10 to produce an aqueous slurry12 contalnlng solid dichloroisocyanuric 'acid (DCCA). The aqueous slurryis separated in separator 14 to solid diehloroisocyanuric acid 16 andbrine cfllueut (first aqueous medium) 18 which is usually discarded. Thesolid dichloroisocyanuric acid 16 is neutralized in neutralizer 20 withsodium hydroxide. The neutralizer cfllucnt is a sodiumdichloroisoeyanurate dihydrate slurry 22 which is separated in separator24 into wet sodium dichloroiso cyanurnte dihydrate particle 26 and asecond aqueous medium 28 containing dissolved sodiumdichloroisocyanurate. The solid dichloroisocyanurate dihydrate is thendried in dryer 30 to remove uncombined water at a temperature from 20'C. to C. to produce dry sodium dichlorolsocyanurate dihydrateessentially free from sodium chloride contamination. The second aqueousmedium 28 is recycled preferably to neutralizer 20. Any liquid build-updue to recycling can be corrected by a purge stream or by evaporation.Furthermore. liquid build-up problems can be avoided by recycling thesecond aqueous medium back to ehlorinator 10.

By such a process the undesirable sodium chloride eoproduct inehlorinator 10, is separated in separator 14,

and leaves the process in the brine efiiucnt 18. in contrast, the priorart process of making a dihydrate from trisotlium cyanuratc and chlorineresults in sodium chloride being cuproduced in ncutralizer 20 andcreates problems both with regards to product purity and stability andalso complicates the recycling of the second aqueous medium.

The reaction in chlorinator i is shown employing cyanuric acid, chlorineand sodium hydroxide as reaclttttls. However, the sodittm hypochlorltecan be used to replace sodium hydroxide ttnd some of the chlorine.

Preferably. dlchloroisoeyanurie acid is continuously produced at atemperature from 5' C. to 45' C. by reacting sodium hydroxide andcyanuric acid in a mole ratio of from l.9:l to 2.i:l along withsuliicicnt chlorine in an aqueous reaction medium to resttlt in aproduct slurry having a pli of from 1.5 to 3.5.

More preferably the chlorination reaction is continuously performed intwo reaction zones by feeding an aqueous slurry containing eyanuric acidand about two moles of caustic soda per tnolc of cyanttric acid. and aseparate stream of chlorine continuously to a first aqueous reactionzone in which the pli is maintained between 5 and 9 and the temperaturebetween 5' C. and 40 C. A portion of reaction mixture from the firstreaction zone is continuously withdrawn and fed. with additionalchlorine. lo a second aqueous reaction zone in which the pH ismaintained between 1.5 and 3.5 and the temperature between 5' C. and C.Reaction product containing solid dichioroisocyanurie acid iscontinuously withdrawn from the second reaction zone. The reactionproduct is usually separated according to standard solid-liquidseparation techniques into solid dichloroisocyanuric acid and liquidctiiucnt (first aqueous medium). The solid dichloroisoeyanuric acid ispreferably washed with water.

The solid (preferably washed) dichloroisocyanuric acid is slurried witha second aqueous medium and neutralized with sodium hydroxide to producean aqueous slurry containing sodium dlchloroisocyanurate dihydrnte. Thisneutralization is carried out at temperatures of from about 5 C. toabout 65' C. at a pit of frotu about 6 to about 7. The reactantsnormally are in about stoichiometric proportions within this pH rangeand are in the fornt of an aqueous slurry containing up to about 50%solids. The aqueous slurry is then separated into its liquid and solidComponents which are referred to as second aqueous medium and soliddichloroisocyanurate dihydratc respectively.

The second aqueous medium is essentially saturated with di\ tOiVCddichloroisocyanurate and therefore an cflieicnt continuous processrequires the recovery of these tiichioroisocyanurats values by recyclingor reusing the second aqueous medium. The amount of dichloroisocyanuratcin the second aqueous medium depends upon temperature and in commercialoperation the temperature varies between 0' C. and 40 C. which resultsin dichloroisocyanuntte concentrations of between about lti% and aboutin the second aqueous medium.

The solid sodium dichloroisocyanurate dihydrate after being separatedfrom the second aqueous medium is quite pure because there isessentially no sodium chloride in the second aqueous medium.

The sodium dichloroisocyanurate dihydrate, after being separated fromthe second aqueous medium, is dried to remove at least 95% of theuncombined water employlag conventional drying apparatus but at acritical tempcraturc of 50' C. or lower preferably from 15' C. to 50' C.More preferably, 99% or more of the uncombined water is removed. Thepressure during drying is not critical with about atmospheric pressurebeing preferred for convenience, however, subatmospheric pressure isacceptable.

The solid sodium dichloroisocyanurate dihydrate product can be dried toremove some of the combined water to re ult in a mixture of thedihydratc and monohydrate and/or anhydrous dichloroisocyanuratc. Such aproduct mixture having an average H O content of greater than EXAMPLEThis exemplifies the continuous production of sodiumdlchloroisoeyaaurate dihydrate beginning with two-sta e. continuouschlorination of cyanuric acid to produce dichloroisocyanuric acidfollowed by neutralization of the acid with sodium hydroxide to producesodium dichloroisocyanurate dihydratc and then drying thedichlorosiocyanurale dihydrate to remove uncombined water.

For chlorination two reactors were used. Each reactor contained a sidearm for continous overflow of product slurry. Vent gas from the secondstage reactor \va.-. passed to tile first stage reactor. Vent gas fromthe first slttgo reactor was passed to a scrubber.

The run was started with a heel of water in the first stage reactor. TheNaOli-cyanuric acid feed was prepared by mixing dry cyanuric acid withsufficient 6% NaOH solution to provide a NaOH/cyanuric acid mole ratioof 2.02. This slurry was fed at a Constant rate of about l2 ml. perminute to the first stage reactor along with the separate addition ofsufitcicnt chlorine to mainlttin a pH of 7 in the first stage reactor.After the first stage reactor filled and started overflowing to thesecond stage reactor. chlorine was fed to the second stage reactor tomaintain a pii of 2.5. The volumes of reaction mixture in the first andsecond reactors were about 500 and i250 ml.. so that the averageretention times were about 42 and 104 minutes. respectively.

After about three hours, both reactors were operating at essentiallysteady-state conditions, which were a tempcrature of IS C. and pH of 7.0and 2.5, respectively. During the succeeding period of 3 hours and 20minutes while these conditions were maintained, the amount of cytmurlcacid fed to the first stage and the product over- [low from the secondreactor were measured. The product slurry was filtered at essentiallythe operating temperatom of 15' C. and 292 grams of product on the drybasis were recovered. This product contained 71% available chlorirteshowing it was essentially pure ichloroisocyanurlc acid. '1 he filtrate(first aqueous medium) was discarded.

The continuous preparation of sodium dichloroisocyanurate dihydrate wascarried out by neutralizing in a third reactor a 20% by weightdiehioroisoeyanuric acid aqueous slurry produced with thedichloroisocyanuric acid product obtained above.

The dichioroisocyanuric acid aqueous slurry was fed into the thirdreactor and simultaneously a 50% sodium hydroxide solution was fed intothe third reactor at a rate sufficient to maintain the pH at 6.8. Thetemperature of the reaction mixture was controlled between 20 and 25' C.by cooling the reaction mixture in a water cooled heat exchanger. Theresulting slurry containing solid sodium dichloroisocyanurate dihydratesalt was continually removed and centrifuged to separate the salt fromthe second aqueous medium. The second aqueous medium was returned to thethird reactor as make-up reaction medium.

The wet salt product was gently dried in a warm air stream at about 40'C. to remove only the unbound water, leaving essentially last the twobound waters of hydration. The resultant free-flowing white crystallinedihydrate was assayed for available chlorine (found 55.2%. theoretical55.4%). Furthermore. the dihydrate was not contaminated with sodiumchloride.

What is claimed is:

1. A continuous process for producing sodium dichioroisucyanuratedihydrate without the coproduction of sodium chloride in the sameaqueous medium comprischlorinating cyanuric acid with suiiiclentchlorine and an alkali selected from the group consisting of sodiumhypochlorite and sodium hydroxide to produce solid dichloroisocyanuricacid in a first aqueous medium at a pH of from 1.5 to 3.5 and at atemperature 01' from 5' C. to 45' C.. the mole ratio of said alkali tosaid cyanuric acid being from 1.90:1 to 2.1 i,

separating the solid dichloroisocyanuric acid from the first aqueousmedium,

neutralizing the solid dichloroisoeyanuric acid with sodium hydroxide ina second aqueous medium at a pii from 6 to 7 and a temperature 015' C.to 65 C. to form sodium dichioroisocyanurate dihydrate particics,

separating the sodium dichioroisocyanurate dihydrate particles from thesecond aqueous medium, recycling the second aqueous medium, and

drying the sodium dichloroisocyanurate dihydrate particles at atemperature from 15' C. to 50' C. to remove at least 95% oi theuncombined water and to produce a tree-flowing. particulate, sodiumdichioroisocyanurate dihydrate product essentially tree of so diumchloride contamination.

2.11m process 01 claim 1 in which the second aqueous medium is recycledto the neutralization step.

3. The process of claim 1 in which the second aqueous medium is recycledto the chlorination step.

4. The process of claim 1 in which the drying temperature is at about 40C. and at least 99% of the uncombined water is removed.

5. The process of claim 1 in which during drying at least 99% of theuncombined water is removed and some of the combined water is removed toproduce sodium dichloroisocyanurate dihydrate mixed with sodiumdichlorolsocyanurate particles havin less than two waters of hydrationsuch that the water content of the mixture is greater than 11%.

6. The method of claim 1 wherein the chlorination comprises feeding astream of an aqueous slurry, containing cyanuric acid and two mois ofsodium hydroxide per moi of cyanuric acid, and a second stream ofchlorine continuously to an aqueous reaction zone in which the pH ismaintained between 5 and 9 and the temperature between 5' C. and 40 C..continuously withdrawing a portion of the reaction mixture and decdingit, with additional chlorine, to a second aqueous reaction zone in whichthe pH is maintained between 1.5 and 3.5 and the temperature betweenabout 5' C. and 20 C., continuousiy withdrawing reaction product fromthe second reaction zone and recovering the precipitateddichloroisocyanuric acid from the withdrawn reaction product.

References Cited UNITED STATES PATENTS JOHN M. FORD, Primary Examiner

